In reading through comments on YT videos on this subject, one of the comments in support of a pistol-mounted WML is that since it would only be used in those rare (if ever) circumstances when it's legally justified to draw the pistol, there is less worry about discovering a dead WML due to a discharged cell from other use. This is yet another advantage of a dedicated fight light, regardless of how it's carried.
This IMO is a serious consideration if only one "EDC light" is carried that's used for all of those mundane EDC tasks, and being depended upon should a more dire situation suddenly present itself.